Organic polyisocyanates have been reacted with compounds having active hydrogen groups, such as hydroxyl groups, to produce a wide variety of useful urethane containing materials such as coatings, hot-melt adhesives, moldings and materials used in injection molding applications and composite or laminate fabrications. Urethane bonds are used ubiquitously in polymer chemistry to produce a wide variety of useful compositions. Typical of the art is the patent to Markle et al, U.S. Pat. No. 5,097,010.
The urethane bond is conveniently obtained by the addition reaction of an isocyanate group (either an aliphatic or an aromatic isocyanate) and an aliphatic alcohol or an aromatic (also known as aryl) hydroxyl group (also known as a phenolic group). The urethane bond is formed between the oxygen atom of the hydroxyl group and the carbon atom of the isocyanate group. An alternate term often used is “urethane linkage”. This reaction is reversible at sufficiently high temperatures as indicated by showing the following reaction as an equilibrium process. In this equation, R is alkyl or aryl and R′ independently is alkyl or aryl. The equilibrium constant K is defined as k1/k2 where k1 is the rate constant of the forward, or urethane bond forming reaction, where k2 is the rate constant of the reverse reaction involving reformation of RNCO and R′OH. These rate constants each vary as a function of the temperature, with k1 and k2 both increasing as the temperature increases. However, k1 will dominate (i.e., k1>>k2) over some temperature range between ambient temperature and some intermediate higher temperature since the forward reaction typically has a lower activation energy than the reverse reaction. As a result of these activation energy differences, k2 will increase more rapidly than k1 as the temperature is increased. Thus, at some higher temperature, k2 may equal k1 (where the equilibrium constant K=1) and may in certain cases become appreciably greater than k1 at still higher temperatures. Hence, the equilibrium constant will range from quite high values at ambient temperature but can become relatively smaller at sufficiently high temperatures so that significant and useful concentrations of isocyanate groups will be present.
The forward, or urethane bond forming reaction, can be affected by simply heating an equimolar mixture of isocyanate and hydroxyl groups to the temperature at which k1 is large enough that urethane bond formation occurs in an acceptable, or practical, period of time (from a few minutes to several hours). Catalysts, such as tertiary amines or certain organotin compounds, can speed both the forward and reverse processes, but are not necessary to bring about the urethane bond forming reaction or the establishment of equilibrium. If both compound types are difunctional, that is, if they are diisocyanates and dialcohols or diphenols, the forward reaction will produce polymeric products (polyurethanes) of very high molecular weights. The achievable molecular weight of fully reacted (i.e., of essentially non-reversed) pairs will be limited by the presence and concentration of monofunctional isocyanates or monofunctional alcohols; by the isocyanate concentration and the dialcohol or the diphenol concentrations not being equal to each other; or, by the intervention of adventitious impurities which deplete the amount of either isocyanate or hydroxyl groups by side reactions. However, as the temperature of the polyurethane is further increased and k2 increases faster in comparison to the increase in k1, significant and measurable reverse reaction to isocyanate and either alcohol or phenol will occur. The approximate reversion onset temperatures of urethanes derived from representative combinations of aliphatic or aryl isocyanates and alkyl or aryl hydroxyl groups (as defined earlier) have been previously reported by Z. W. Wicks, Jr., “Blocked Isocyanates” Progress in Organic Coatings, 3, pp. 73–99 (1975) as shown in Table 1 below:
TABLE 1Approximate UrethaneReversion OnsetIsocyanate TypeAlcohol TypeTemperature (° C.)Aryl (e.g. MDI)aAryl (e.g. Phenol)120Alkyl (e.g. HDI)bAryl (e.g. Phenol)180 (118)cAryl (e.g. MDI)Alkyl (e.g. n-Butanol)200Alkyl (e.g. HDI)Alkyl (e.g. n-Butanol)250aMDI = 4,4′-diphenylmethane diisocyanatebHDI = 1,6-hexamethylene diisocyanateca wide variation of reversion onset temperatures exists in the literature for urethanes prepared from aliphatic isocyanates and phenolic compounds, the lowest being 118° C. (M. Gedan-Smolka, Thermochimica Acta, 351, pp 95–105 (2000).)
These reversion onset temperatures are approximate values which represent the onset of reversal or a temperature where the practical effect of reversal, such as the onset of distillation or evaporation of a phenolic compound or an alcohol from a heated mixture occurs, or where infrared (IR) spectroscopy of heated samples indicates the onset of isocyanate and alcohol or phenol formation from a previously unreversed urethane compound.
Crosslinking in polymers is known to improve their physical properties and increase mechanical properties (such as but not limited to tensile and flexural strengths and moduli). Typically, crosslinked polymers do not melt or dissolve in solvents (for the uncrosslinked polymers). Hence they cannot be melt or solution processed. However, if crosslinks are present that contain at least one thermally reversible bond, the polymer should maintain the advantageous properties associated with crosslinking while below the reversion onset temperature of such crosslinks, but should be readily either melt or solution processable at some temperature above the reversion onset temperature.
In the work described herein, it was sought to identify combinations of particular diisocyanates or polyisocyanates and dialcohols or diphenols, or polyalcohols or polyphenols, which would possess reversibility of practical utility (described further below) in terms of some relatively high temperature at which onset of reversion would occur. This would allow the preparation of polymers with both backbone urethane bonds (i.e. urethane bonds as part of the structure of the long molecular strands constituting a polymer chain), and crosslinking urethane bonds (i.e. urethane bonds connecting two of the long molecular strands constituting a polymer chain with bridging bonds, which result in dramatic increases in average molecular weight, such as for example a doubling thereof) which might be expected to have practical utility up to, or very close to, the urethane reversion onset temperature as described above. If sufficient reversible bonds, including crosslinks, are incorporated into such a reversible bond-containing polymer structure, polymers may be formed at some elevated temperature, by first heating the mixture of reactive components to some temperature above the practical reversion onset temperature such that a mixture of molten, or dissolved, partially assembled, urethane bond-containing, polymer fragments is established. Such mixture will be easily stirrable, have a low viscosity, and can be melt processed by methods such as melt spinning of fibers and fabrication of components by injection molding and extrusion processing. It will also be solution processable, provided the mixture is heated in a solvent which dissolves both the starting components and partially assembled, but uncrosslinked components. For example, both dry and wet fiber spinning of fibers are possible. As this mixture is cooled below this reversion onset temperature, the isocyanates and hydroxyl functional groups will recombine to reform urethane bonds providing a high molecular weight and crosslinked polymer structure having the advantages provided by the original crosslinks.
At low to moderate levels of crosslinking, polymers that are nonmeltable and nonsoluble in solvents which readily dissolve the uncrosslinked polymer will be obtained. However, at lower levels of crosslinking these crosslinker polymers will either be inherently tacky or will become tacky when heated above their softening temperature (Tg) and will swell in solvents (for the uncrosslinked polymer), while at intermediate levels of crosslinking these properties (tackiness at room temperature, or the occurrence of the increase in tackiness, when heated above Tg, and swelling in solvents) will begin to decrease to the point where they are no longer observed. When inherently soft polymers (i.e. ones which are above their Tg, or softening temperature, but below the Tm, or melting, temperatures) are crosslinked at low to moderate levels they will exhibit elastomer properties. That is, they will be extensible under low to moderate pulling stress but will resist extension with forces that can become considerable (i.e. moderate to high tensile strength) depending on the actual level of crosslinking and the other molecular properties of the polymers. When the extension stress is released these polymers will retract to their original unstressed dimensions.
Historically, elastomers were known for many years as homopolymers (such as natural rubber, or high molecular weight cis-1,4-polyisoprene) or random copolymers (such as styrene-co-butadiene in which the butadiene is the major component and is present as a randomly distributed combination of 1,4- and 1,2-butadiene units, with the 1,4-form dominating). Further, these polymers only achieved commercial viability in such valuable end uses as automobile tire treads or carcasses, radiator hoses, or fan belts, and so on, when the polymer molecules were chemically crosslinked with strong covalent, or in some cases, ionic, bonds which were nonreversible once formed. Such elastomers may have crosslinks between as few as one or two per 1000 polymer backbone building blocks (i.e. monomer units incorporated into the backbone) or as many as four or five per 100 polymer backbone building blocks. However, more typically such elastomers may have from five to ten crosslinks per 1000 polymer backbone building blocks (i.e. monomer units incorporated into the backbone) up to one to three per 100 polymer building blocks. The optimal level of crosslinking for desired elastomeric properties will vary somewhat as a function of both polymer variables such as molecular weight and molecular weight distribution and the particular elastomer mechanical properties desired.
Additionally, reinforcing fillers (such as carbon black, clays, silica and so on) have also been found to be either necessary, or very useful, in concert with the crosslinking, to provide the desired end use properties. More recently, starting in the decade of the 1960's, it was discovered that strong, highly elastic, thermoplastic (i.e. meltable and melt processable) elastomers were possible when soft segment polymer molecules, i.e. ones with a subambient Tg, and hard segment polymer molecules, i.e. ones with both Tg and Tm above ambient temperature, were covalently joined together in appropriate sequences and relative molecular masses. For example, block copolymers in which soft, high cis,-1,4-butadiene or cis-1,4-isoprene polymer block sequences were attached to two anchor, or external, polystyrene block sequences, were found to be tough elastomers, even without the addition of reinforcing fillers such as carbon black or fumed silicon. Yet these block copolymers, consisting of polystyrene-cis-1,4-diene-polystyrene (A-B-A block copolymers) in which the polystyrene blocks are, by convention, A blocks and the cis-1,4-diene blocks are by convention B blocks) melt when heated above the melting point (Tm) of the polystyrene (i.e. hard, A) blocks. Kraton™ was the first, and is still one of the important, commercial products of this class. The soft, subambient Tg polydiene blocks must be the internal or B block and the structure must be at least A-B-A, although it can be more extended, e.g. A-B-A-B-A or higher, or branched, or star structures, so long as the hard A blocks are the ends or anchors. The molecular weights of both the hard A and soft B blocks must be above some minimum values, and the molecular weight of B somewhat greater than two times that of A, to achieve useful elastomer mechanical properties. The hard (A) blocks provide virtual crosslinks by forming phase separated domains, or solid, micron-sized aggregates, of a number of polystyrene molecules. These act as rigid anchors or tie points which allow the surrounding soft, extensible polydiene molecules to undergo deformation or movement relative to each other, i.e. be stretched or extended some finite amount under mechanical stress, until the maximum amount of chain unfolding and relative chain movement (strain) has occurred and further movement would require breaking covalent bonds or pulling apart the solid polystyrene domains. The stress at this point is the maximum stress, or strength, possible, before yield or failure. As this stress is released the polydiene chains will tend to return to their starting configurations. The stress/strain properties at a given temperature, say normal ambient temperatures of about 20–25° C., and the degree of retention of these properties upon repeated application of some maximum stress, depend, in addition to the molecular weight parameters already discussed, on the softening and melting properties of the hard block polymer and on the degree of completeness and order of the phase separation of the A and B blocks. As temperature is increased, the stress/strain properties and their retention upon repetition are rapidly depleted.
If some method could be provided to further strengthen, or stabilize, the phase separated hard phase, the stress/strain property would be expected to be enhanced, but especially as the temperature is raised. In particular the loss of stress/strain repeatability (i.e. hysteresis), and the undesirable increase of such properties as compression set, might be greatly reduced. The application of reversible crosslinking to both these classes of elastomers is discussed briefly below. It should be noted and emphasized that A-B-A type block copolymers may be based on a variety of polymer A and polymer B types, so long as the above discussed criteria for hard A block and soft B block properties are met. However, in these thermoplastic block elastomers, the reversible covalent crosslinks should always be provided in the hard A blocks. Furthermore, the crosslink density, when confined to the hard A blocks, may be much higher than that which is normally incorporated in ordinary, or nonblock structure, elastomers, as described earlier above.
Examples of such polymers include, but are not limited to, polyurethanes (i.e. polymers that have urethane backbone connecting bonds) or they may be polymers with other backbone repeating units, such as aliphatic polyesters, acrylic polymers or copolymers, polyolefins (such as ethylene propylene copolymers), styrene butadiene copolymers with about 70 or more weight percent of butadiene content, or polymers containing unsaturation in the backbone such as poly-cis-1,4-polyisoprene or poly-cis-1,4-polybutadiene. If the crosslinks provided in any of these soft polymers are urethane bonds with thermally reversible properties, then the crosslinked polymer products will be elastomers between the lower temperature Tg and the higher reversion onset temperature of the urethane crosslinking bond.
However, if the polymers are of the special block structure type in which a soft, extensible polymer (block B) with a subambient Tg is covalently linked to a hard, high melting polymer (block A), that possess Tm's well above ambient temperature, such that a repeating A-B-A-B-etc. block copolymer structure is obtained, and in which the reversible urethane crosslinks are now provided in the hard blocks (A), rather than the soft blocks (B), thermoplastic elastomers with the added feature of covalent crosslinks between repeat segments of the high melting, or hard block (A) portion, will be obtained. This will result in increases in the tensile and flexural strengths and moduli, and other mechanical properties, of the non-covalently crosslinked thermoplastic elastomers. Of particular importance, these increased mechanical properties will be obtained up to appreciably higher temperatures than in the non-covalently crosslinked thermoplastic elastomers. As a result, large, commercially valuable improvements in the mechanical properties and use temperature ceilings of such covalently crosslinked thermoplastic elastomers can be expected.
Higher levels of reversible urethane crosslinking within polymers are expected to show great utility in terms of mechanical (such as tensile or flexural) strength, rigidity (i.e. very high modulus values), scratch or abrasion resistance, resistance to organic solvents or water at various pH values, and other important properties, when used in practical applications. Some but not all applications include molded parts, composite structures (e.g. elastomers, glass fiber or fabric, carbon fiber or fabric, various particulate filled structures, and the like), coatings on various substrates such as metals, glass reinforced moldings or composites, ceramics, silicon wafers or electronic components, and high structural strength adhesives.
The need exists for new materials having improved melt processing and end use characteristics. The present invention addresses those needs.